1. Field of the Invention
The present invention relates to a flexbeam for a helicopter and, more particularly, to a flexbeam of a hub structure provided with no bearing members.
2. Description of the Related Art
Blades of a helicopter are coupled with a rotor shaft through a hub so as to enable each of flapping motion, feathering motion and lead-lag motion. In order to enable these three motions on the blades, a hub structure with no bearing members, using an elastic (flexible) flexbeam has been recently proposed and put to practical use.
The flexbeam used in such a hub structure with no bearing members has flexible flapping portions, lead-lag portions and feathering portions. The three motions on the blade are achieved by deflection of the flapping portions and of the lead-lag portions, and torsion of the feathering portions.
There has been proposed such a conventional flexbeam in which the feathering portion has two substantially Y-shaped cross sections coupled to each other (refer, for example, to Patent document 1: Japanese Laid-Open Patent Publication No. Tokukai-hei 10-287297 (page 3 and FIG. 4)). The feathering portion of the flexbeam described in Patent document 1 includes a center member disposed at the center in a beam thickness direction, unidirectional members the center portions of which are disposed so as to get contact with upper and lower surfaces of the center member, and front edge portions and rear edge portions of which are formed and bent to be in parallel with the center portions, and laminated members disposed on the surfaces of the.
However, in the feathering portion of the flexbeam described in Patent document 1, since flat portions of the unidirectional member have a uniform plate thickness while the bending portions are partially thicker, the torsion rigidity at the bending portions still results in high. This causes concentration and increase of shear stress due to torsional deformation, and therefore there is plenty of scope for improvement of increasing beam strength. The high torsion rigidity of the flexbeam also requires a large control force for changing a blade pitch, which produces a load on a control system for changing the pitch. This load causes a problem of reducing the safety and reliability of the constructing parts for the control system.
The flexbeam described in Patent document 1 has the laminated members for reinforcement disposed on the bending portions of the unidirectional members in the feathering portions, however, this reinforcement still could not solve the reliability problem in the control system, caused by the high torsion rigidity.
An object of the invention is to provide a flexbeam in which reduction of torsion rigidity at feathering portions improves strength of the flexbeam by reducing shear stress due to torsional deformation.
Another object of the invention is to improve safety and reliability of a control system for changing a blade pitch.
In order to achieve the above-described object, in accordance with a first aspect of the invention, the flexbeam having flapping portions and lead-lag and feathering portions, in which each lead-lag and feathering portion comprises: a central portion having an approximate plate-like shape with an elongated and narrow width; middle portions each of which has an approximate plate-like shape and which continuously connects to the central portion and branches from both ends of the central portion extending along the longitudinal direction thereof, and extends upward and downward; and edge portions each of which has an approximate plate-like shape and which continuously connects to the middle portion and bends from each end of the middle portions and extends almost in parallel with the central portion, wherein at corners formed on at least one of branch portions and bending portions, concaves each of which has a cross-section of an approximate arc shape and which continuously and smoothly connects to two surfaces forming each of the corners and is formed inside extended planes of the two surfaces.
According to the first aspect of the invention, since the lead-lag and feathering portion has concaves each of which has a cross-section of an approximate arc shape and which continuously and smoothly connects to two surfaces forming each of the corners and is formed inside extended planes of the two surfaces, at corners formed on the branch portions between the central portion and the middle portion and/or at corners formed on the bending portions between the edge portions and the central portion, it is possible to reduce torsion rigidity of the feathering portions and thereby to moderate force concentration.
Accordingly, since the shear stress caused by torsional deformation is reduced, it is possible to thereby improve the beam strength. Reduction of the torsion rigidity in the lead-lag and feathering portion also reduces the control force for changing the blade pitch. As a result, this structure reduces the load on the control system for changing the pitch and improves the safety and reliability of components constructing the control system.
In accordance with a second aspect of the invention, the flexbeam having flapping portions and lead-lag and feathering portions, in which each lead-lag and feathering portion comprises: a first composite material extending in a beam length direction and having a plurality of fiber orientations; and second composite material extending in the beam length direction and having the fiber orientation in the same direction as the beam length direction, the first composite material and the second composite material being joined integrally, wherein the first composite material comprises: a center portion arranged at an approximate center in a beam thickness direction and extending in a beam width direction; first extending portions that extend in a beam front edge direction and a beam rear edge direction so as to branch from both ends of the center portion to beam upper and lower directions; and second extending portions that extend in the beam front edge direction and the beam rear edge direction so as to bend almost in parallel with the center portion at ends of the first extending portions, and the second composite material comprises: upper-and-lower portions arranged on upper and lower surfaces of the center portion, of the first extending portions and of the second extending portions, respectively; and front-and-rear edge portions arranged on beam front edge and rear edge side surfaces of the first extending portions and the second extending portions, wherein concaves are formed on at least one of branch portions formed by the center portion, the first extending portions, the upper-and-lower portions and the front-and rear edge portions, and at bending portions formed by the first extending portions, the second extending portions, the upper-and-lower portions and the front-and-rear edge portions.
According to the second aspect of the invention, since in each lead-lag and feathering portion, concaves are formed at branch portions formed by the center portion and the first extending portions, which are formed of the first composite materials, and the upper-and-lower portions and the front-and rear edge portions, which are formed of the second composite material, and/or at bending portions formed by the first extending portions and the second extending portions, which are formed of the first composite materials, and the upper-and-lower portions and the front-and-rear edge portions, which are formed of the second composite material, the torsion rigidity can be reduced and the force concentration can be moderated.
As a result, this structure reduces the load on the control system for changing the blade pitch, and improves the safety and reliability of components constructing the control system.
In the first or second aspect of the invention, preferably, the flexbeam further comprises a doubler made of composite material, for covering at least one of the concaves.
According to the flexbeam having such a structure, since the doublers reinforce the corners (branch portions and/or bending portions) the plate thickness of which is thinned by the concave, the beam strength can be improved against bending loads and torsional loads.
The doubler may be made of a sheet type of fiber reinforced composite material which is made from a prepreg obtained by impregnating a woven reinforced composite fiber material with a thermosetting resin.
Preferably, the portion having an approximate plate-like shape has a minimum cross-sectional thickness at the concave portion, which is in a range of 80-90% of a thickness at a flat region thereof. The reason for this is that excess depth of the concave reduces the strength of the lead-lag and feathering part, and insufficient depth of the concave does not reduce the torsion rigidity of the part.
In the second aspect of the invention, preferably, the first composite material is a glass fiber reinforced one which comprises one or more sheets of prepreg obtained by impregnating a woven glass fiber with a thermosetting resin, e.g., epoxy resin. The second composite material may be loop material in which glass fibers are extended in a direction and impregnated with a thermosetting resin, e.g., epoxy resin.
Preferably, the flexbeam further comprises a third composite material covering surfaces of the portions comprising the first and second composite material. The third composite material may be a glass fiber reinforced one which comprises one or more sheets of prepreg obtained by impregnating a woven glass fiber with thermosetting resin.